The present invention relates to arrangements for drying plate-shaped wood products in general, and more particularly to arrangements for drying veneers, plywood plies and the like.
There are already known various constructions of drying arrangements for plate-shaped wood products, among them such which are capable of drying the wood products in a continuous operation during the passage of such products through the respective drying arrangement.
In drying arrangements of this type, the plate-shaped wood products have to be supported and advanced during their passage through the drying arrangement in such a manner as to assure effective drying of such products, on the one hand, and to prevent deformation of and/or development of undue stresses in such wood products during their drying, on the other hand.
The known continuously operating drying arrangements are usually constructed either as roller driers or as belt driers. The roller drier utilizes as its supporting and advancing means a series of pairs of cooperating rollers distributed over the length of the drying arrangement. Then, the plate-shaped wood products are advanced through the drier between the upper and the lower roller of the respective roller pairs. An advantage of the roller drier resides in the fact that, because of the adjustable line pressure existing between the upper and the lower roller of the respective roller pair, the plate-shaped wood product is kept planar.
However, the roller drier is not particularly suited for drying relatively thin, wet wood products, which have only limited stiffness. More particularly, inasmuch as there is provided no guidance for the wood product between the two adjacent roller pairs, a secure transfer of the wood product from one roller pair to the other is not assured.
Belt driers are better suited for thin plate-shaped or sheet-shaped wood products, such as veneers. In such driers, the thin plate-shaped wood products are advanced in the desired path through the drier between the lower run of an upper belt conveyor and the upper run of a lower belt conveyor. In this construction, the guidance of the wood products is unproblematical. As a result of the resting of part of the weight of the upper conveyor belt on the wood product, there is even obtained a certain smoothing effect, which may be sufficient for thinner plate-shaped wood products. Naturally, this smoothing action is considerably less pronounced than that occurring in roller driers.
A drier for wood products which combines the roller and belt conveyors along the path of passage of the wood products through the drying arrangement is known from the German Pat. No. 758,580. In this conventional construction, the belt conveyor section is arranged at the upstream, and the roller conveyor section at the downstream, portion of the path. One of the purposes of this arrangement of the conveyors is to also utilize the advantageous smoothing action of the rollers of the roller conveyor for thinner plate-shaped wood product. More particularly, it was established that, after a certain pre-drying, even the relatively thin plate-shaped or sheet-shaped wood products possess a sufficient degree of stiffness, so that they can be advanced through and by the roller conveyor, without encountering any problems.
The present invention is concerned predominantly with those problems which arise when relatively thick plate-shaped wood product, especially peeled veneers or plies of, for instance, poplar wood, which are frequently being used as the internal plies in the manufacture of plywood, are to be dried. Plies or veneers of this kind have previously been dried almost exclusively in roller driers. However, difficulties are often encountered when this approach is taken, especially when the wood products are to be dried to a final moisture contents of approximately between 1 and 3%. This is attributable to the fact that the pressure exerted by the rollers on the wood products during the drying operation interferes with the ability of the wood product material to shrink. Owing to this interference, shrinking stresses develop in the wood product material, which often result in the development of cracks in or other destruction of the wood products.
The German Pat. No. 630,132 deals with this problem as encountered in roller conveyors. In this arrangement, the roller conveyor has roller pairs only along a short stretch of the path of passage of the plate-shaped wood products through the drier, at the leading end of this path. Each of the roller pairs includes an upper roller and a lower roller, and one of the main purposes of such roller pairs is to securely draw the plate-shaped wood products into the drier and to assure their proper progression through the drying arrangement. On the other hand, along the predominating part of the path, the length of which as considered in the advancement direction is a multiple of the length of the introducing section of the path flanked by the rollers of the roller pairs, there are disposed only single rollers which merely support the plate-shaped wood product from below. In the drier of this construction, the smoothing action of the roller pairs in the introducing section is quite insignificant, particularly in view of the relatively small length of the introducing section, while no smoothing action whatsoever takes place in the main section where the wood product is merely loosely supported from below by the single rollers.